clarify explanation of normalization of the contact matrix

**Is your feature request related to a problem? Please describe.**

clarify the explanation of the [normalization step](https://epiverse-trace.github.io/tutorials-late/contact-matrices.html#normalising-the-contact-matrix-to-ensure-the-correct-value-of-r_0). 

**Describe the solution you'd like**

It was clarified by using the next-generation matrix.

**Additional context**

No.